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A client uses the REGISTER method to register the address listed in the To header field with a SIP server.
A user agent MAY register with a local server on startup by sending a REGISTER request to the well-known "all SIP servers" multicast address "sip.mcast.net" (224.0.1.75). This request SHOULD be scoped to ensure it is not forwarded beyond the boundaries of the administrative system. This MAY be done with either TTL or administrative scopes [25], depending on what is implemented in the network. SIP user agents MAY listen to that address and use it to become aware of the location of other local users [20]; however, they do not respond to the request. A user agent MAY also be configured with the address of a registrar server to which it sends a REGISTER request upon startup.
Requests are processed in the order received. Clients SHOULD avoid sending a new registration (as opposed to a retransmission) until they have received the response from the server for the previous one.
Clients may register from different locations, by necessity using different Call-ID values. Thus, the CSeq value cannot be used to enforce ordering. Since registrations are additive, ordering is less of a problem than if each REGISTER request completely replaced all earlier ones.
The meaning of the REGISTER request-header fields is defined as follows. We define "address-of-record" as the SIP address that the registry knows the registrand, typically of the form "user@domain" rather than "user@host". In third-party registration, the entity issuing the request is different from the entity being registered.
To: The To header field contains the address-of-record whose registration is to be created or updated.
From: The From header field contains the address-of-record of the person responsible for the registration. For first-party registration, it is identical to the To header field value.
Request-URI: The Request-URI names the destination of the registration request, i.e., the domain of the registrar. The user name MUST be empty. Generally, the domains in the Request-
URI and the To header field have the same value; however, it is possible to register as a "visitor", while maintaining one's name. For example, a traveler sip:alice@acme.com (To) might register under the Request-URI sip:atlanta.hiayh.org , with the former as the To header field and the latter as the Request-URI. The REGISTER request is no longer forwarded once it has reached the server whose authoritative domain is the one listed in the Request-URI.
Call-ID: All registrations from a client SHOULD use the same Call-ID header value, at least within the same reboot cycle.
Cseq: Registrations with the same Call-ID MUST have increasing CSeq header values. However, the server does not reject out-of-order requests.
Contact: The request MAY contain a Contact header field; future non-REGISTER requests for the URI given in the To header field SHOULD be directed to the address(es) given in the Contact
header.
If the request does not contain a Contact header, the registration remains unchanged.
This is useful to obtain the current list of registrations in the response. Registrations using SIP URIs that differ in one or more of host, port, transport-param or maddr-param (see Figure 3) from an existing registration are added to the list of registrations. Other URI types are compared according to the standard URI equivalency rules for the URI schema. If the URIs are equivalent to that of an existing registration, the new registration replaces the old one if it has a higher q value or, for the same value of q, if the ttl value is higher. All current registrations MUST share the same action value. Registrations that have a different action than current registrations for the same
user MUST be rejected with status of 409 (Conflict).
A proxy server ignores the q parameter when processing non-REGISTER requests, while a redirect server simply returns that parameter in its Contact response header field.
Having the proxy server interpret the q parameter is not sufficient to guide proxy behavior, as it is not clear, for example, how long it is supposed to wait between trying addresses.
If the registration is changed while a user agent or proxy server processes an invitation, the new information SHOULD be used.
This allows a service known as "directed pick-up". In the telephone network, directed pickup permits a user at a remote station who hears his own phone ringing to pick up at that station, dial an access code, and be connected to the calling user as if he had answered his own phone.
A server MAY choose any duration for the registration lifetime. Registrations not refreshed after this amount of time SHOULD be silently discarded. Responses to a registration SHOULD include an Expires header (Section 6.20) or expires Contact parameters (Section 6.13), indicating the time at which the server will drop the registration. If none is present, one hour is assumed. Clients MAY request a registration lifetime by indicating the time in an Expires header in the request. A server SHOULD NOT use a higher lifetime than the one requested, but MAY use a lower one. A single address (if host-independent) MAY be registered from several different clients.
A client cancels an existing registration by sending a REGISTER request with an expiration time (Expires) of zero seconds for a particular Contact or the wildcard Contact designated by a "*" for all registrations. Registrations are matched based on the user, host, port and maddr parameters.
The server SHOULD return the current list of registrations in the 200 response as Contact header fields.
It is particularly important that REGISTER requests are authenticated since they allow redirecting future requests (see Section 13.2).
Beyond its use as a simple location service, this method is needed if there are several SIP servers on a single host. In that case, only one of the servers can use the default port number.
Support of this method is RECOMMENDED.
Posted by yogendra
REGISTER Messaging is basically Used by a client to log in and register its address with a SIP
registrar server.
Posted by sagitraz
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